Software transactional memory (STM) is a concurrency control mechanism analogous to database transactions for controlling access to shared memory in concurrent computing. A transaction in the context of transactional memory is a piece of code that executes a series of reads and writes to shared memory. In other words, a transaction accesses data in one or more objects. An object in the context of transactional memory is a set of connected memory locations that are locked as one entity. An object in this context might also be a static variable, or a set of such variables, or it might be a set of cache lines.
STM is used as an alternative to traditional locking mechanisms. STM allows concurrent programs to be written more simply. A transaction specifies a sequence of code that is supposed to execute as if it were executing in isolation. This illusion of isolation is achieved by fine-grained locking of objects, and by executing in a mode that allows the side-effects of the transaction to be rolled back if the transaction is discovered to be in conflict with some other transaction. We say that a data access is “transacted” if the code generated for that access has been modified to include support for these locking and rollback mechanisms.
Transactions may be nested, and can be classified as open or closed nested. If a thread is currently executing a transaction and reaches the start of a new atomic block, this atomic block is executed as a closed nested child transaction of the currently-executing parent. This nested transaction executes within the same isolation boundary as the enclosing transaction, and just like other memory accessed within the enclosing transaction, the effects of the nested transaction will only become visible when the enclosing transaction commits. In other words, the parent transaction is effectively suspended, and the closed nested transaction is allowed to run to completion before processing in the parent is resumed. When a nested transaction rolls back, its temporary effects are undone and the state of the parent transaction is restored to the point that the nested transaction began.
The “outermost” transaction being executed by a given thread is not nested; we call this the top-level transaction. This top-level transaction must execute atomically, so the nested transactions become part of it. Nesting could arise, for example, if some abstractions A and B each had internal representation invariants they wanted to maintain even in use by concurrent threads, and they therefore used atomic blocks in the implementations of their methods to guarantee that these invariants are not violated by concurrent accesses. Now assume that some higher-level abstraction C uses instances of A and B in its implementation, and has some invariant that relates these A and B instances. Methods of C might use transactions to ensure that this invariant is not violated. If A and B methods are used inside C's transactions, the transactions in the A and B methods will be nested (in this use).
Current transactional memory systems do not allow work performed within the isolation boundary of one transaction to be distributed between multiple concurrent threads of execution. In current systems, a transaction may have only one nested child transaction. The semantics of such systems simply do not allow such parallelism within a transaction, and attempts to execute more than one nested transaction at a time would result in nested transaction log entries intermixed without order in the parent's log and other errors, and a breakdown of the basic underlying fine grained locking protocols used to provide the illusion of isolation.